1. Field of the Invention
The present invention relates generally to an optical network, and in particular, to an optical network based on a wavelength division multiplexing (WDM) technology.
2. Description of the Related Art
As a WDM technology for transmitting a plurality of wavelengths through a single-strand optical fiber and its related fabrication techniques, including routing, switching, and adding/dropping of optical signals in a ring network, have dramatically improved, it is now possible to build an optical network, which provides a super high-speed transmission.
In general, a WDM optical network is divided into a mesh network consisting of optical cross connectors (OXCs) and a ring network consisting of optical add/drop multiplexers (OADMs). In the mesh network, as each network node is connected to a plurality of optical fibers, the switching mechanism used to address the link failure is complex and tend to degrade the recovery rate. However, in the ring network, as an optical add/drop multiplexer and a network node is connected to a 2-strand or 4-strand optical fiber, the switching mechanism is simple. Therefore, the ring network is more widely implemented.
A node provided in the WDM optical ring network can be comprised of optical add/drop multiplexers with an optical switching element for adding or dropping an optical signal, and switching devices for performing the protection-switching of the network. The ring network uses a 2-strand or 4-strand optical fiber corresponding to the number of input/output optical fibers. In addition, the ring network can be divided into a unidirectional transmission network and a bidirectional transmission network.
FIG. 1A illustrates a structure of a general WDM optical ring network, and FIG. 1B is a diagram illustrating the switching operation to address the link failure. In particular, FIGS. 1A and 1B illustrate the structure of a bidirectional optical ring network having a 4-strand optical fiber. The conventional WDM optical ring network comprised of a 4-strand optical fiber for transmitting an optical signal bidirectionally uses a link protection switching method based on a loop-back principle. As shown, a first outer ring network 4 of the optical ring network is used during a normal operation. If there is a link failure in the first outer ring network 4, a second outer ring network 3 of the optical ring network is used to perform the bypass of the optical signals based on the loop-back principle. In this regard, 2×2 optical switching devices 110a–180a are connected to the first outer ring network 4 and the second outer ring network 3 at both ends of the optical add/drop multiplexers 10a–40a. Similar to the configuration of the outer ring networks 3 and 4, the inner ring networks 1 and 2 1 also include a network 1 for a normal operation and a network 2 for recovery switching operation. Likewise, 2×2 optical switching devices 110b–180b are provided for the loop-back operation and also connected to the first inner ring network 1 and the second inner ring network 2 at both ends of optical add/drop multiplexers 10b–40b. 
In the conventional optical ring network consisting of a 4-strand optical fiber, as the outer ring networks 3 and 4 and share no resource with the inner ring networks 1 and 2, it is possible to set the wavelengths used for transmission of an optical signal to the same value. That is, the outer ring transmits optical signals having wavelengths of λ1, λ2, λ3, . . . , λN, and the inner ring also transmits optical signals having wavelengths of λ1, λ2, λ3, . . . , λN. If a bidirectional transmission is provided, the outer ring transmits the optical signals clockwise, while the inner ring transmits the optical signals counterclockwise.
In the event of a link failure, a switching is performed by looping back the optical signal using the two 2×2 optical switching devices located on both sides of the failed link in the opposite direction. For example, as illustrated in FIG. 1B, if a failure occurs in an optical link of the network 4, which connects an optical add/drop multiplexer OADM1a (10a) to an optical add/drop multiplexer OADM2a (20a), optical signals of λ1, λ2, λ3, . . . , λN, which were transmitted from the optical add/drop multiplexer OADM1a (10a) to the optical add/drop multiplexers OADM2a (20a), are looped back to the second outer protection switching ring network 3 through an optical switching device sw12 (120a), thus traveling in a counterclockwise direction through the second outer ring network 3. Ultimately, the optical signals of λ1, λ2, λ3, . . . , λN transmitted through the second outer protection switching ring network 3 are delivered to the optical add/drop multiplexer OADM2a (20a) via an optical switching device sw21 (130a).
As illustrated by the optical switching device sw12 (120a) of FIG. 1A, when a optical transmission link normally operates, the 2×2 optical switching device is in a bar (parallel) state, so that a signal applied to an input#1 (i1) is delivered to an output#1 (o1), and a signal applied to an input#2 (i2) is delivered to an outpt#2 (o2). However, when a failure occurs in the optical link, the optical switching device is in a cross state, causing signal applied to an input#1 (i1) to be delivered to an output#2 (o2), and a signal applied to an input#2 (i2) is delivered to an output#1 (o1). At this time, an optical switching device which is not adjacent to the failed link, is still in a bar state.
As explained above, for a bidirectional self-healing optical ring network to work, a 4-strand optical fiber is necessary, thus complicating and increasing the cost of the network. Therefore, there is a need to provide an improved network that is simple and less expensive to implement.